Don't use postfix both hold the expression and act as a stack to evaluate the expression. I would suggest declaring an array of type double called myStack to use in evaluating the expression.

The elements of postfix are all type char. Since the compiler recognizes type char by the integer value assigned to it in some character set, it is possible/legal to add/subtract two char variables, though you have to know what you are doing to make it do what you intend it to. It's best if you convert a given char to an int before doing routine mathematical calculations. This can usually be done by subtracting char zero from whatever char you have. This works when characters 0 through 9 are listed sequentially in the character set used, but most character sets do that.

Your char by char evaluation of the expression means that evaluating the expression will be most straight forward when any numerical input is limited to single digit values. Your char by char approach could be extended to more sophisticated expression, but it would take some work. If you want to allow more than single digit integers or decimal values to be used as part of the input expression then viewing the expression as a series of substrings separated by some delimitng value (the space char would be a convenient delimiting value) would probably make more sense. Each substring can be separated/parsed/viewed a stand alone string which is convenient since strings can pretty readily be determined to represent numerical values or mathematical operators and if it is a string representing a numerical value it can be converted to that numerical value by using a stringstream. Conceiveably the substrings could represent more than just (multidigit) numerical values and mathematical operators, of course, but being able to do that is probably beyond what you were asked to do.

I have a 2d matrix with dimension (3, n) called A, I want to calculate the normalization and cross product of two arrays (b,z) (see the code please) for each column (for the first column, then the second one and so on).
the function that I created to find the ...

Write a C program that should create a 10 element array of random integers (0 to 9). The program should total all of the numbers in the odd positions of the array and compare them with the total of the numbers in the even positions of the array and indicate ...